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NERC Responsive Mode Project: A Thermocline Nutrient Pump

Project Partners and Duration

NERC Responsive Mode research project NE/F002432/1 ("A Thermocline Nutrient Pump") was jointly awarded to Bangor University (Principal Investigator: Dr. Tom Rippeth) and the Proudman Oceanographic Laboratory [since April 2010, part of the National Oceanography Centre] (Principal Scientist: Prof. Jonathan Sharples, since January 2010 based at the University of Liverpool). The project runs from 2008 to 2013.

Background

The seasonal thermocline in temperate shelf seas acts as a critical interface in the shelf sea system. It is a physical barrier to vertical exchange, controlling biological growth through the summer and enabling the sequestration of atmospheric CO ₂ .

Once the spring bloom is over, the seasonal thermocline separates the sun drenched but nutrient depleted surface waters from the dark nutrient rich deep water. The vertical mixing of nutrients across the seasonal thermocline acts to couple this well-lit surface zone with the deep water nutrient supply, leading to the formation of a layer of phytoplankton within the thermocline (the subsurface chlorophyll maxima). This phenomenon is estimated to account for about half of the annual carbon fixation in seasonally stratified shelf seas, and yet the controlling physics is only just being unravelled. The identification and parameterisation of the physical processes which are responsible for the vertical mixing of nutrients across the thermocline is a vital prerequisite to the understanding of shelf sea ecosystems.

Project Aim

This project aim is to investigate the role of wind driven inertial oscillations in driving vertical mixing across the seasonal thermocline, identifying the mechanisms and processes responsible for their generation and dissipation on both special and temporal scales. This is to be achieved through an observational campaign closely integrated with numerical model predictions using both 1D and 3D numerical models.

Specific Hypotheses to be tested:

1. A significant fraction of the total turbulent mixing at the thermocline is the result of episodic spikes in inertial shear.

2. Inertial shear events are sensitive to local stratification and tidal conditions.

3. Inertial mixing events have significant horizontal variability.

Project Objectives

1. Acquisition of long time series of well-resolved current and temperature profiles to quantify the episodic nature of the shear events and investigate their relationship with surface winds, tides and stratification. This objective contributes to testing of Hypothesis 1.

2. Investigate spatial variability of the generation/dissipation of inertial shear events caused by horizontal variations in wind forcing, tidal characteristics and horizontal patchiness in density structure. This contributes to testing of Hypothesis 2.

3. Determination of the spatial connectivity of inertial shear events as a mechanism for non-local generation, transfer and dissipation of intertial energy. This contributes to testing of Hypotheses 2 and 3.

4. Determination of consistent discrepancies between modelled sequences of intertial events and direct observations and identification of possible causes. This will identify deficiencies in the current modelling physics.

5. Investigate the link between inertial mixing and vertical nitrate fluxes into the thermocline and the consequences of incorrect modelling of the episodic nature of this mixing process.

Fieldwork Schedule

Key:

VMP: Vertical Microstructure Profiler
CTD: Conductivity, temperature and depth sensor
ADCP: Acoustic Doppler Current Profiler
SAPS: Stand-alone Pump Sampling
VMADCP: Vessel Mounted ADCP
LADCP: Lowered ADCP
RAS: Remote Access Sampler